Organic foams are commonly made by introducing a blowing agent (e.g., a supercritical fluid such as CO2 or freon) into a polymer. The polymer is subjected to a rapid pressure drop which causes the blowing agent to form bubbles in the polymer. This process creates a solid containing gas bubbles—namely a foam.
Ceramic foams may be constructed from a variety of materials and may be used in various applications such as thermal insulation, separation processes, catalysis and low dielectric constant materials. In simplified terms, a ceramic foam is a foam where the solid phase is composed of a ceramic material.
The most common method of producing a ceramic foam involves the impregnation of an organic polymer foam (e.g., polyurethane) with a ceramic slurry. The coated organic polymer is dried, then the organic phase burned off. After a sintering step, the resulting ceramic foam is a replica of the original organic precursor.
Another method of producing a ceramic foam, named high internal phase emulsion (“HIPE”) involves the preparation of a concentrated emulsion containing a continuous phase of a polymerizing monomer (e.g., sodium silicate) that is dispersed in a pore-forming phase (e.g., petroleum spirit) with the aid of a surfactant. The continuous phase is stabilized by polymerization, washed, and then dried to obtain the foam.
Both of the methods described above produce open-cell ceramic foams. However, these methods do not use the combination of a gas and a liquid phase that is used in blowing agent foam production methods.
Cellular silica and SiC-whisker-reinforced cellular silica have been produced using physical blowing agents incorporated into a ceramic suspension. (see Fujiu et al., J. Am. Ceram. Soc., vol. 73, pp. 85-90 (1990) and Wu et al, J. Am. Ceram. Soc., vol. 73, pp. 3497-3499 (1990), respectively). This process uses a stabilized aqueous suspension of colloidal silica. The blowing agent is dispersed as small liquid droplets in the suspension with the aid of a surfactant and methanol. The pH of the suspension is adjusted to cause gelation, which is accompanied with a rapid viscosity increase. At this stage, the temperature is raised above the boiling point of the blowing agent thereby producing bubbles in the gel and giving rise to the foam. The duration of the viscosity increase and the setting temperature must be carefully monitored at this stage in order to prevent foam collapse.
In another ceramic foam process (P. Sepulveda, Am. Cer. Soc. Bull., 76, 61-65 (1997)), the foam structure is stabilized by the polymerization of organic monomers incorporated into to the ceramic powder suspension. Initiator and catalyst are added to the system after the foaming stage to induce the polymerization of the organic monomer and the setting of the porous structure.
The above methods have several drawbacks. Most of these methods involve a series of steps (e.g., forming the starting compound, adding blowing agents, etc.). This complicates and increases the cost of the foam manufacturing process. Furthermore, the foams produced thus far often have a 70-90% porosity. Accordingly, a need exists for an improved method of producing ceramic foams, with the option of increasing the pore fraction.